1. Field of the Invention
This invention relates to a magnetic fluid seal device which seals bearings and other mechanical rotating portions by magnetic fluid.
2. Description of the Prior Art
FIGS. 3 and 4 are this kind of seal device according to the prior art which are used in the magnetic disc device.
In the drawings, Numeral 1 is a cylindrical housing, and Numeral 2 is a shaft fixed to the housing 1 through bearings 3. "C" is a ring seal member consisting of a ring magnet 6 and ring pole pieces 4, 5 coaxially fixed to the end surfaces thereon. A gap "g" is provided in between the pole pieces 4, 5 and the inner race 3a in the bearing 3. The seal member is fixed by bond to the inner peripheral face of the outer race 3b in the bearing 3. The inner race 3a is fixed to the shaft 2, and the outer race 3b to the housing 1. The inner race 3a and the pole pieces 4, 5 are of magnetic material. Numeral 7 is magnetic fluid kept by magnetic force in the above-mentioned gap "g", or kept by a magnetic force generated by a magnet 6 in between the inner race 3a and the seal member C. Numerals 8, 9, 10 11 and 12 are respectively a disc inserted onto the shaft 2, a magnetic head, balls, a ball retainer and a seal plate.
Since the device is of such construction as described above, the gap between the seal member C and the inner race 3a is sealed with the magnetic fluid 7 kept in the gap, thus preventing dust (grease dust) generated from the bearing from entering the clean zone.
With prior art, however, as shown in FIG. 5, a wide range of ring-shaped groove m defined by the inner side faces 4a, 5a and the inner peripheral face 6a of the magnet 6 is formed at the peripheral portion of the seal member facing the inner race 3a bounded by the magnetic fluid 7. As a result, when magnetic fluid 7 is being injected from one gap "g" at one pole piece 4 using an injector 13, the magnetic fluid would be sometimes filled only in the gap (single sealed), but not in the other gap for the other side of pole piece 5 (not double sealed).
The reason for the single sealing (as the result of the failure of the desired double sealing) may be as follows:
(1) The magnetic fluid 7 is strongly absorbed to the first corner 4m at the side of pole piece 4 and the second corner 5m at the other side of pole piece 5, thus resulting in the stuck fluid which would not be readily moved by the injection pressure.
(2) The width W of the above-mentioned groove m is made narrower by a thinner magnet 6, but an excessively thin magnet would weaken the holding force of the magnetic fluid 7, thereby resulting in a limited thickness of the magnet thick enough to obtain the holding force. Therefore, the width W is made substantially wide, resulting in a long travel distance of the magnetic fluid 7, leading to a substantially high injection pressure required to move the magnetic fluid 7 from the first corner 4m to the second corner 5m.
In this connection, magnetic fluid 7 must be injected from a gap "g" at one side of pole piece 4, because, with the prior art shown in FIGS. 3 and 4, the fluid cannot be injected from the other gap "g" at the other side of pole piece 5.
There has been available another magnetic fluid seal device according to the prior art as disclosed in the Japanese Laid-Open Patent No. 76053/1985 (FIG. 6).
The following is the description of the device in FIG. 6:
The rotating shafdt 2x is made of non-magnetic material. Across the ring permanent magnet 6x, two ring pole pieces 4x, 5x are attached. And, the ends of the ring pole pieces 4x, 5x are inwardly bent facing each other remaining a gap "g" at the peripheral face of the above-mentioned rotating shaft 2x, and also remaining a gap "gx" between the two facing bent portions. This construction intends not only to form a magnetic field acting between the two facing bent tips along the axial direction of the rotating shaft 2x, but also to seal the gap "g" between the two ring pole pieces 4x, 5x and the rotating shaft 2x to keep a desired amount of magnetic fluid 7 within the magnetic field.
With this seal device, however, since the rotating shaft 2x is made of a non-magnetic material, no magnetic field is formed between the rotating shaft 2 and the ring pole pieces 4x, 5x, but a magnetic field is formed only along the small gap "gx" between the facing bent tips of the ring pole pieces 4x, 5x, to keep magnetic fluid. But the keeping of the fluid is performed only at one place around the rotating shaft. Therefore, there may not be any problem in injecting magnetic fluid 7x, but the so-called one stage sealing may not have an adquate seal effect.
As preceding examples according to the prior art as shown in FIG. 6, there have been known the followings: Australia Patent No. 267656, Japan Laid-Open Patent Nos. 166567/1987, 57761/1980 and 146956/1982, Japan Laid-Open Utility Model No. 106575/1978, Japan Patent Publication NO. 13420/1964, England Patent Nos. 1575145, 1292338 and Soviet Patent No. 742657, and the article "Magnetic Fluid Seal" in the magazine "Machine Design" (March 26, 1968) at page 146.
FIG. 7 shows another preceding example according to the prior art shown in FIG. 8 in the Japan Utility Model Publication No. 3579/1987.
The preceding example according to the prior art shown in FIG. 7 is similar to the construction of the prior art shown in FIG. 6 in which the rotating shaft 2x is made of non-magnetic material, but is different from that in that the rotating shaft 2y is made of magnetic material. Therefore, two-stage (duplicated) seal is formed also in the gaps "gap" between the shaft 2y and respective side faces of the bent extensions at the ends of the two ring pole pieces across the ring permanent magnet.
However, when magnetic fluid 7y is injected using an injector like FIG. 5 into one side of gap "gap" between one pole piece 4y or 5y and the rotating shaft, since the fluid cannot directly reach the other side of gap "g", the fluid would move to the other side of gap through the inside of the one gap, the internal peripheral surface of the magnet 6y, and the inside of the other side of pole piece. Therefore, a substantially great amount of magnetic fluid is required in the groove "my" defined by the both pole pieces 4y, 5y and the magnet 6y, thus resulting in an unstable operation.